The present invention relates to the foundry field, and in particular to casting monocrystalline metal parts.
Traditional metal alloys are equiaxed and polycrystalline: in the solid state, they form a plurality of grains of substantially identical size, typically of the order of 1 millimeter (mm), but of orientation that is random to a greater or lesser extent. The joints between grains constitute weak points in a metal part made of such an alloy. The use of additives for reinforcing these inter-grain joints nevertheless presents the defect of reducing the melting temperature, which is particularly troublesome when the parts produced in this way are for use at high temperature.
In order to solve that drawback, columnar polycrystalline alloys were initially proposed in which the grains solidify with a determined orientation. By orienting the grains in the direction of the main load on the metal part, that makes it possible to increase the strength of such parts in a particular direction. Nevertheless, even in parts subjected to forces that are strongly oriented along a particular axis, such as for example turbine blades that are subjected to centrifugal forces, it can still be advantageous to provide greater strength along other axes.
That is why, since the end of the 1970s, new so-called “monocrystalline” metal alloys have been developed that enable parts to be cast that are formed as single grains. Typically, such monocrystalline alloys are alloys of nickel with a concentration of titanium and/or aluminum of less than 10 molar percent (mol %). Thus, after solidification, those alloys form two-phase solids, with an upsilon (γ) first phase and an upsilon prime (γ′) second phase. The γ phase has a face centered cubic crystal lattice in which the atoms of nickel, aluminum, and/or titanium can occupy any position. In contrast, in the γ′ phase, the atoms of aluminum and/or titanium form a cubic configuration, occupying the eight corners of the cube, while the atoms of nickel occupy the faces of the cube.
One of these new alloys is the “AM1” nickel alloy developed jointly by Snecma, les laboratoires de l'ONERA, l'Ecole des Mines de Paris, and Imphy SA. The parts made out of such an alloy can not only achieve particularly high levels of mechanical strength along all force axes, but they also present improved ability to withstand high temperatures, since they do not need any additives for binding their crystal grains together more strongly. Thus, metal parts produced on the basis of such monocrystalline alloys can advantageously be used in the hot portions of turbines, for example.
Nevertheless, even when using such special alloys, it can be difficult to avoid a recrystallization phenomenon during the production of such parts, giving rise once more to crystal grains and to new weak points in the part. In a conventional foundry method, the molten alloy is cast into a cavity in a mold through at least one casting channel in the mold, the mold is removed after the alloy has solidified so as to release the part, and the part is then subjected to heat treatment, such as quenching for example, in which the metal is initially heated in order to be subsequently cooled rapidly so as to homogenize the γ and γ′ phases in the monocrystal without causing it to melt.
Nevertheless, the mechanical impacts to which the parts are subjected after casting can locally destabilize the crystal lattice of the monocrystal. Thereafter, the heat treatment can trigger unwanted recrystallization in the locations that have been destabilized in that way, thereby losing the monocrystalline nature of the part and giving rise to points of weakness therein. Even while making considerable efforts, it is very difficult to avoid mechanical impacts in the handling of molds that may weigh several tens of kilograms, particularly since removal of the mold of itself involves the use of mechanical blows. Furthermore, on its own, a limited reduction in the temperature of the heat treatment does not make it possible to prevent those recrystallization phenomena significantly.